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The effect of n=1(2201) buffer layers on n=2(2212) and n=3(2223) thin films of Bi2Sr2Can-1CunOywas investigated to reduce the strain caused by the lattice mismatch between the films and the substrates. The bilayer thin films of 2223/2201 and 2212/2201 were grown epitaxially on SrTiO3(100) substrates by dc sputtering. When the thickness of the buffer layer was 10-25 nm, the superconducting transition temperature increased by ?20 K for 2223/2201 and by ?6 K for 2212/2201. The increase was attributed to the reduction of the strain from the experimental results of the full width at half-maximum of X-ray diffraction peaks, the length along the c-axis and the roughness of the film surface.